January 11, 2000
This document was prepared by the Steering Committee of the Save Lake Davis Task Force, with biological assessments by California Department of Fish and Game biologists serving on the Committee and information from out-of-state biologists familiar with northern pike.
During the August 5, 1999, Save Lake Davis Coalition Meeting in Portola, the public was asked for suggestions for possible northern pike control measures. Those ideas, along with proposals from the Task Force's Steering Committee, including Department biologists, were compiled into an initial list of approximately 40 options. This list was provided to the Steering Committee on October 28, 1999, and to the public at the Coalition Meeting on November 3, 1999. The list was organized into three major categories; biological methods, physical methods and chemical methods. Of the proposed methods, the use of formulated rotenone (rotenone with agents added to aid dispersion) and other chemicals prohibited by Proposition 65 were excluded from use at this time by action of the Director of the Department of Fish and Game.
The following analyses are subject to revision as new information is obtained. These analyses provide information for the public, particularly those persons who took the time to share their ideas to help solve a difficult problem. They also identify those methods that the Steering Committee and the Department have determined offer the greatest potential for control of northern pike at Lake Davis. It is apparent that a combination of methods, perhaps not individually effective enough to provide a solution to the pike problem, will be needed to achieve containment and control. The primary considerations in these efforts are preventing the northern pike population explosion experienced at Lake Davis from 1994 to 1997 and precluding the escape of pike from the lake.
The Steering Committee grouped the various alternatives into three categories - options that should be implemented, options needing further information or evaluation and options not recommended for implementation. The following analyses follow these groupings. The Department is supportive of those options recommended for use when employed in combination. Some experimental control measures involving a combination of barrier nets, encircling nets, electrofishing and detonation cord are also proposed. These were developed as the result of a Steering Committee meeting and are generally consistent with Steering Committee preferences. Also, included is the discussion of a new aquatic exclusion system ("Gunderboom") that may be used at the lake.
Northern pike were found in two tributaries above existing road structures which are currently barriers to pike. This suggests that barriers may be ineffective in keeping northern pike from upstream areas. The placement of well-constructed barriers, however, should be effective in the overall control effort. Barriers can also be used to retain fish in upstream areas for eventual removal.
Northern pike have been reported to spawn in the marshy areas of lakes or in connected sloughs. Pike are able to move into these areas with the development of sufficient clearance between inshore ice and the bottom to provide access to streams or marshy areas (Franklin and Smith 1963). It would be difficult to block all potential spawning areas in Lake Davis. Blocking off the primary spawning areas once fish have entered them, however, is an approach currently being used at Mosquito Slough thought to be an important spawning area. Fish so blocked can then be taken by various means. Placement of barriers to separate pike from desirable spawning areas or encircling them by nets in spawning areas in other areas of the lake for removal (Freeman and Cow creeks) is consistent with the effort to preclude a substantial pike population increase.
Northern pike predation on salmonids has been documented in western waters (Jones 1990, McMahon and Bennett 1996, Horner 1999), and observed at Lake Davis (pers comm, Ivan Paulsen, Senior Fishery Biologist, California Department of Fish and Game). Reductions in stocking fingering trout will reduce the total pounds of prey available to northern pike and should reduce northern pike growth rates. The stocking of catchable-sized trout should continue, however, to maintain a quality trout fishery at Lake Davis.
Angling is often used to manage fisheries by saving particular groups of fish and restructure fish populations. Ricker (1968) described techniques or options typically associated with angling used to regulate fisheries. None of the methods, however, provided any insights for significant reduction of northern pike at Lake Davis. Goeman et al (1993) reported that northern pike removal in small Minnesota lakes through trap-netting and angler harvest was ineffective in altering population size structure. The Department has not found examples where sportfishing has eradicated an undesired fish species. There are, however, ample examples in the literature of the impact of overfishing on fish populations - hence the need for bag limits. While the use of fishing to remove northern pike from Lake Davis is not the solution to the problem, it is a tool that can be used in combination with other methods to effect suppression. Derbies are fishing contests that encourage the take of a particular fish and generally offer an inducement for the most fish or the biggest fish.
Encircling nets include beach or haul seines, and various types of commercial open water nets. Beach and haul seines are typically used in shallow waters and target fish species that concentrate near shore. To prevent fish from escaping the net, a lead line is affixed to the bottom of the net to keep it in contact with the bottom and not become entangled in obstructions. As such, seines are most effective on smooth, shallow, bottoms with no obstructions (Hayes 1996 et al.). Northern pike occupy areas with dense aquatic vegetation and seines typically "roll" on the bottom allowing fish to escape. Some nets are effective for pelagic (open water) species and would not readily catch near-shore and vegetation oriented species such as northern pike. While the Department believes that employing seines to capture northern pike probably would not result in a substantial reduction in pike numbers, their use is viewed as another tool in a control effort integrating various methods singularly not offering the ultimate solution. The use of encircling nets will be further discussed under B. Experimental Control Measures. Static seines (seines anchored in place) can be used in near shore areas with vegetation to assist in the trapping and capture of pike.
Trawls are funnel-shaped nets that are towed along the bottom or in the water column. As the net is towed through the water, fish entering the net eventually tire and move towards the rear of the net. They are the most commonly used sampling gear in oceanic and estuarine habitats but are also used extensively in large lakes. Commercial trawls are typically employed in the ocean. Although trawls can be effective at catching fish, they have limitations. Trawls cannot be effectively used where there are objects on which they can get caught. In freshwater these can be aquatic plant beds, rocky outcroppings, or submerged objects. These types of habitats are difficult to sample. Trawls also require a relatively powerful vessel to pull them along the bottom or through the water column and as such, are not suitable for shallow water habitats along the shore (Hayes et al 1996). Based on a review of the habitats occupied by northern pike at Lake Davis, the Department has concluded that the use of trawls would be difficult to use but consistent with a goal of containment.
Entanglement nets include gill and trammel nets. Trammel nets consist of parallel panels of suspended netting. Fish are captured in a bag or pocket of the netting. They are most efficient when set around an aggregation of fish. Gill nets are effective at catching a variety of fish species including northern pike. They have been reported to be most effective during the early spring spawning period. Gill nets have been fished in Lake Davis and based on highest capture rates, it would require 100 nets fished for one month to capture 1,000 northern pike. This estimate, however, is not based on fish being concentrated in areas during spawning activities. Gill and trammel nets can be used as part of the Department's containment effort.
Electrofishing is an important tool used in the Department's control and monitoring efforts. Its application will also be discussed under B. Experimental Control Measures. Based on previous field sampling, northern pike are vulnerable to electrofishing.
Electrofishing is traditionally a sampling tool and should not be viewed singularly as an eradication method. Substrates and aquatic vegetation affect the ability to electroshock fish. Mud, silt and abundant aquatic vegetation are common in the near shore areas of Lake Davis and reduce electrofishing efficiency. Northern pike are difficult to see in vegetation and may become trapped in vegetation, preventing capture. In addition, aquatic vegetation and emergent sedges prevent boats from accessing all habitat areas. Electrofishing efficiency is influenced by a number of factors including vulnerability of the fish and water conductivity.
Electrofishing can be used in combination with other methods as a containment tool. This is especially true when fish are congregated in areas for spawning or protective cover. Electrofishing is the use of electricity to capture fish. An electrical field is generated in the water and fish are stunned allowing capture. Boat electrofishing has been used at Lake Davis to allow capture with a hand-held net. Electrical fields produced by boat mounted electrofishers typically electroshock northern pike in a 4-8 foot radius from the anode (positive) probes and at a maximum depth of 6-8 feet. Fish outside the field or in deeper waters are not electroshocked and larger fish are able to avoid capture.
Directing fish through the use of electricity is one of the earliest uses of electricity in fisheries management. While northern pike are vulnerable to both electrofishing and gill nets, the ability of northern pike to evade the electrofishing electrical field and gill nets suggests that this technique will not likely result in a substantial reduction in northern pike. This approach was attempted during monitoring efforts in 1999 and resulted in two northern pike captured. This method could be employed during the period fish congregate for spawning. Also, other creative approaches combining nets and electrofishing could be attempted.
A variety of trap and fyke nets have been designed to capture fish. Trap nets have been reported to be effective for capturing northern pike (Rod Pierce, Minnesota Department of Natural Resources, Save Lake Davis Steering Committee meeting minutes, August 25, 1999). Trap nets were fished during the summer and fall of 1999 and three northern pike were captured. Based on average northern pike capture rates, it would require almost 8,000 traps fishing for one month to capture 1,000 northern pike. Capture rates are expected to be higher during the early spring spawning period. This method can be considered another containment tool.
Proposal 10: Stock Brown Trout as Predator Species
The Department has not found documented evidence that any fish species could be an effective control for northern pike through predation. In general, food habitats studies indicate that northern pike are, in fact, predators on species suggested for control (Lagler 1965, Frost 1954, Allen 1939, Hunt and Carbine 1950, Lux and Smith 1960, Lagler 1956). Although predation is a natural occurrence in aquatic communities, the Department does not believe that this factor could result in the elimination or substantial reduction of northern pike. The Department is aware that the brown trout is an effective predator on other fishes. Since catchable-sized trout will be stocked at the lake to maintain the trout fishery, the Department will include brown trout as part of its plantings. While brown trout will not control northern pike, any pike taken by brown trout would be considered additive to the containment effort.
1. Barrier Net at Mouth of Mosquito Slough to Contain Concentrated Pike
Commencing November 3, 1999, the Department increased its electrofishing efforts in the Mosquito Slough area to remove fish that might spawn in the spring and to confirm that northern pike were concentrated in the area. From November 3 to November 23, ninety-one fish were collected in the Mosquito Slough area, including a 27-inch fish. Also, 181 of the 197, fish taken by electrofishing efforts at the lake came from the Mosquito Slough area.
On December 2, a barrier net was placed across the mouth of Mosquito Slough. The intent of this action was to take advantage of an opportunity to contain as many pike as possible while they were concentrated in this area. Areas upstream of Mosquito Slough, including Big Grizzly Creek and the flood plain likely to be covered with water from spring run-off, contain prime pike spawning habitat. Mosquito Slough contains excellent nursery habitat for larval pike. Both areas were separated from the lake by the barrier net.
b. Intended Use of Barrier
The principle intent of the barrier net is to set up the conditions under which a considerable number of adult, juvenile and larval pike may be removed from the lake. Prior to spring spawning, some pike will be removed by other pike. The product of spring spawning will be larval pike . These small fish, up to about two inches in length, will move from the Big Grizzly Creek spawning areas downstream to the nursery areas of Mosquito Slough. Movements of these larvae will coincide with spawning times. Prior to these fish moving downstream to the barrier net, an additional net with finer mesh will be installed adjacent to the existing net. Adult and juvenile pike remaining in the containment area can be expected to feed on larval pike. At the most opportune times, detonation cord can be used to remove pike of all ages. Detonation cord has been widely used elsewhere to remove unwanted fish of all sizes, including larval forms (see discussion of detonation cord in II. Options Requiring Additional Information or Evaluation). Other removal methods including electrofishing and netting will also be used in the containment area.
2. Placement of Barrier Nets at Mouths of Freeman and Cow Creeks
Barrier nets can be placed at the mouths of Freeman and Cow creeks immediately after the spring thaw. These nets would be used in a manner similar to the use of the Mosquito Slough barrier.
3. Use of Encircling Nets in Conjunction with Monitoring Efforts
Following the rediscovery of northern pike at Lake Davis in May 1999, the Department conducted a systematic sampling program to establish baseline information on the distribution and abundance of pike at the lake. These procedures would be repeated in 2000 to provide an index of relative abundance and distribution of pike.
The Department is proposing to utilize the 2000 monitoring program, mostly electrofishing, to escalate removal of pike from Lake Davis. When individual fish or "pockets" of fish are located by the electrofishing surveys around the lake, the specific areas would be identified for removal actions. When water depths permit, encircling nets would be placed around these areas and fish so contained would be removed by additional electrofishing or by the use of detonation cord.
It is illegal to apply any piscicidal (fish-killing) compound not registered for that use with the appropriate government regulatory agency. In the United States, only four compounds are currently registered for use in fisheries; rotenone, antimycin, TFM, and Bayluscide. The latter two are for sea lamprey sampling or control. Several toxicants not registered for use are used in other parts of the world including pyrethroids and copper sulfate, and there are many other compounds that are toxic to aquatic life (i.e. chlorine). Antimycin is an antibiotic produced by mold that can cause fish mortalities. It is generally recognized as potentially more effective than rotenone, especially for stream treatments. Under some conditions and forms it might be useful for control of northern pike in Lake Davis. Antimycin is generally shipped as a concentrate and mixed on site with an acetone-based solvent. The concentrate can also be bound to sand particles to suit specific situations. Antimycin, however, is not currently registered for use in California.
While the proposed chemicals are not currently registered for use, efforts should be made to investigate what impacts they might have at the lake at the concentrations needed for effective removal of pike or pike eggs. If any of these chemicals are determined to be safe for use, efforts could be made to seek registration as a piscicide.
Fish can be effectively killed by concussion which produces a pressure wave in water. The distance fish can be killed is determined by the size of the explosive and distance the pressure wave travels in the water. Because of extensive shallow water northern pike habitat areas at Lake Davis, numerous explosive charges over a large area might have to be set to be effective. Detonation cord is a commonly used method in fisheries management to create underwater explosions and is easily installed over large areas. This alternative is being explored as a control method. Approval of the Department of Water Resources and the U.S. Forest Service would be required for the use of detonation cord at the lake. The Material Safety Data Sheet for detonation cord provided by the Ensign-Bickford Company, who manufactures the explosive, includes no Proposition 65 chemicals. The Department is currently gathering information on the make-up of any residues left in the water and the air following the use of detonation cord. These residues are likely oxidation products in the form of oxides of nitrogen and nitrates. The Department has requested results from independent chemical tests conducted on detonation cord residues. It is hoped that detonation cord will be deemed totally safe for possible use as part of the control effort. It is the strongest tool available for the direct take of large numbers of northern pike.
Northern pike are known to be associated with aquatic vegetation and larval density can be related to vegetation (Franklin and Smith 1963). Vegetation provides a substrate for spawning and cover for juveniles and adults. Published literature suggests that in their native range, northern pike populations have been depressed by habitat modifications associated with shoreline development and loss of aquatic vegetation. Vegetation control that favors the growth of grasses, sedges, or rushes with fine leaves has been suggested as a northern pike spawning enhancement method (Franklin and Smith 1963). A partial control of aquatic vegetation at Lake Davis would most likely not result in a substantial reduction in northern pike populations. The lake topography and shallow water areas provide ample area for aquatic vegetation growth. Reductions of aquatic vegetation in the lake would reduce the amount of nursery habitat and cover for juvenile pike. Northern pike occupying tributary habitats would not, however, be affected by removal efforts in the vegetated areas of the lake. The use of vegetation removal needs to be explored as to its feasibility on a scale large enough to be effective, but could be viewed as another tool used toward containment.
A "Gunderboom" is a patented, full-water-depth filter curtain comprised of treated polypropylene/polyester fabric suspended by flotation billets on the waters surface and secured in place with anchoring systems. This system is custom designed and deployed to provide for the passage of large volumes of water while excluding fish, fish larvae and fish eggs. This system could be employed at the dam spillway or as a blocking or barrier device.
Dr. William Cox, Department Senior Fish Pathologist, reported (personal communication November 1999) that he is unaware of any pathogen that causes complete mortality of northern pike. He is familiar with a virus called "pike fry rhabdovirus" described by Wolf (1988). The virus was first reported from the Netherlands in 1971 as a cause of high mortality in cultured northern pike fry and subsequently shown to be viral in nature in 1973. The virus has not been found in adult pike but if eggs are exposed to the virus the hatching fry develop the disease in approximately 14 days followed by death. Dr. Cox reported that the source of the virus is speculative but it had only been observed in Europe. It has additionally been recovered from cyprinids (carps and minnows).
Dr. Cox expresses extreme reservations regarding introduction of this virus (or any other pathogen) into California waters. The fact that "pike fry rhabdovirus" causes severe mortality in juvenile pike indicates that it is pathogenic and the number of other species that might be susceptible is unknown. Additionally, viruses can mutate and change the nature of their pathogenicity over time. Because of these concerns, the Department does not recommend the use of any pathogen for control of northern pike at Lake Davis. It will, however, continue to monitor new developments in this area.
Draining the lake is probably not a feasible control tool. The existing dam structure allows for the release of a maximum of only about 230 cubic feet per second (cfs). During certain periods of the year, inflow exceeds outflow. Currently water is released against an energy dissipating wall covered with a grate. This grate is effective in killing fish at the velocity the water strikes it. Department of Water Resources (DWR) personnel estimate that draining the lake may require as long as two years. To drain the lake any faster would require altering the dam structure. This would be expensive and could lead to the escape of pike from the lake. Draining the lake would also drastically impact the trout fishery for a period of time, and this would have considerable economic impact to the local community.
Verbal information from DWR representatives and a review of the inflow and outflow of Lake Davis suggests that it is not feasible to adequately manipulate lake levels in such a way as to negatively affect northern pike spawning or summer habitat. Manipulating a drawdown of the lake would not provide a solution to the pike problem. Pike would survive in upstream areas and in watered vegetation. This option has the same inherent problems as the drain the lake option.
Fish require dissolved oxygen in the water to survive. Reduction or elimination of dissolved oxygen in the water sometimes results in fish kills. This usually occurs during the summer months when water temperatures are high, aquatic plant growth is high, and algal blooms have been substantial. A reduction in the amount of sunlight reaching the water results in an algae and plant die off with resulting decomposition. The decomposing vegetation consumes the oxygen in the water, resulting in a fish kill.
Oxygen depletion would require simulating a natural fish kill by stirring up the organic material deposited on the bottom of the lake and depleting the dissolved oxygen from the water. It is unlikely that this method would result in a total fish kill. Species such as trout would more likely be affected while species such as northern pike, sunfish, and golden shiners with lower dissolved oxygen requirements would likely not be affected. It is doubtful that de-stratification (mixing of waters from various levels) could be accomplished on a scale necessary to create a fish kill at Lake Davis. Northern pike occupying tributary habitats would not be affected.
Sterilization of fish has centered around the development of polyploid chromosomes (multiple chromosomes as opposed to the normal paired chromosome). This technique has successfully produce sterile fish that are incapable of successful reproduction and involves a processes to stress or "shock" fish eggs immediately after fertilization during the first phases of cell division. Polyploid chromosome fish do not develop sexual products and are not induced to complete the spawning act. The introduction of a sterile organism into a population as a control method relies on the organism mating with a fertile organism with the resultant spawning act not producing progeny. Present technology for producing sterile fish results in fish that are incapable of producing offspring and, and more importantly, do not attempt to spawn. Planting sterile northern pike would simply result in increased northern pike in the lake.
Fishing with spears and bowfishing is most effective in calm, shallow waters, or through the ice (Hayes 1996). These methods would be an ineffective control method due to the difficulty in physically reaching all areas occupied by northern pike (i.e. extensive marshy habitats), inability to take large numbers of fish and would not result in a substantial reduction in pike numbers.
For this method, towers or similar devices would be erected at Lake Davis to direct storm generated electricity to the lake, thereby killing the pike by electroshock. This method is considered infeasible due to extreme hazards and uncontrollable nature of creating enough electrical output with sufficient amperage to cause fish mortalities. The Department is also unaware of any technology for "capturing" storm generated electricity as a power source.
Frogs are not known predators of fish eggs or juvenile pike. Frogs form only a small portion of potential diet for pike and their elimination would have little or no impact on pike with possible harm to other aquatic resources.
This option is not acceptable. Allowing the northern pike to take over Lake Davis would result in the destruction of a valuable trout fishery, upon which the local economy heavily depends. Also, it would dramatically increase the likelihood of the northern pike finding its way to other state waters, including the Sacramento-San Joaquin Delta. The impact there to state and federally-listed species would be devasting. Also, the likelihood for great monetary loss and destructive impacts on fisheries resources is great should this option be allowed to occur.
At this time, the Steering Committee does not support any chemical treatment as an option for removal of northern pike from Lake Davis. As a matter of record, the only piscicide (fish-killing chemical) registered for use in California is rotenone. In his letter of October 7, 1999, to Plumas County, the Director of the Department stated that no Proposition 65 chemical would be used to treat Lake Davis. Proposition 65 chemicals are those known to be carcinogenic or have reproductive effects. Tests of formulated (dispersant added) and powdered rotenone have shown that both contain Proposition 65 chemicals.
Allen, K.R. 1939. A note on the food of pike (Esox lucius L.) In Windermere, J. Animal Ecol. 8:72-75.
Franklin, D.R. and L.L. Smith Jr. 1963. Early life history of Northern Pike, Esox lucius L., with special reference to the factors influencing the numerical strength of year classes. Trans. Am. Fish. Soc. 92:91-110.
Frost, W.E. 1954. The food of pike, Esox lucius, L., in Windermere. Jour. Animal Ecol. 23:339-360.
Goeman, T.J., P.D. Spencer, and R.B. Pierce. 1993. Effectiveness of liberalized bag limits as management tools for altering northern pike population size structure. NAJFM 13:621-624.
Hayes, D.B., C.P. Ferreri, and W. Taylor. 1996. Active fish capture methods. Pages 193-220 in B.R. Murphy and D.W. Willis, editors. Fisheries techniques, second edition. American Fisheries Society, Bethesda, MD.
Horner, N. 1999. Biological and social impacts of the illegal introduction of northern pike into Northern Idaho. Management Implication of co-occurring native and introduced fishes, pages 171-173.
Hubert, W.A. 1996. Passive capture techniques. Pages 157-192 in B.R. Murphy and D.W. Willis, editors. Fisheries techniques, second edition. American Fisheries Society, Bethesda, MD.
Hunt, B.P. and W.F. Carbine. 1950. Food of young pike, Esox lucius, L., and associated fishes in Peterson's ditches, Houghton Lake, Michigan. Trans. Am. Fish. Soc. 80:67-83.
Jones, T.S. 1990. Flood plain distribution of fishes of the Bitterroot River, with emphasis on introduced populations of northern pike. Masters thesis. University of Montana, Missoula.
Lagler, K.F. 1956. Freshwater Fishery Biology. 2nd Edition, Wm. C. Brown Co. Publishers, Dubuque, Iowa.
_____________. 1965. The food of pike Esox lucius, in Heming Lake Manitoba. J. Fish. Res. Bd. Canada 22:1357-1377.
Lux, F.E. and L.L. Smith. 1960. Some factors influencing seasonal changes in angler catches in a Minnesota lake. Trans. Am. Fish. Soc. 89:67-79.
McMahon, T.E. and D.H., Bennett. 1996. Walleye and northern pike: boost or bane to northwest fisheries? Fisheries Vol 21(8):6-13.
Ricker, W.E. 1968. Methods for assessment of fish production in fresh waters. F.A. Davis Co. Philadelphia. 349 p.
Wolf, K. 1988. Fish Viruses and Fish Viral Diseases, Cornell University, NY.